Search Images Maps Play YouTube News Gmail Drive More »
Sign in
Screen reader users: click this link for accessible mode. Accessible mode has the same essential features but works better with your reader.

Patents

  1. Advanced Patent Search
Publication numberUS5661825 A
Publication typeGrant
Application numberUS 08/532,126
Publication dateAug 26, 1997
Filing dateSep 22, 1995
Priority dateSep 22, 1995
Fee statusLapsed
Publication number08532126, 532126, US 5661825 A, US 5661825A, US-A-5661825, US5661825 A, US5661825A
InventorsCornelis Van Dam, Helmut Heidrich, Michael Hamacher, Carl Weinert
Original AssigneeU.S. Philips Corporation
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Integrated optical circuit comprising a polarization convertor
US 5661825 A
Abstract
An integrated optical circuit comprises a first device and a second device, which devices are connected by a polarization convertor. The polarization convertor has one or more curved sections of a waveguide, integrated in the optical circuit. The conversion ratio is determined in part by the radius of curvature of the curved sections.
Images(2)
Previous page
Next page
Claims(7)
We claim:
1. An integrated optical circuit comprising:
a first device having an output for emitting radiation with a first state of polarization;
a second device having an input for receiving radiation with a second state of polarization different from the first state of polarization;
a waveguide connecting the output and input of the first and second devices, respectively; and
a polarization converter comprising one or more sections of the waveguide wherein there is a change in the radius of curvature.
2. The integrated optical circuit as claimed in claim 1, wherein a minimum radius of curvature of the curved section is less than 100 μm.
3. The integrated optical circuit as claimed in claim 1, wherein the curved section is a deep etched waveguide structure.
4. The integrated optical circuit as claimed in claim 1, wherein said polarization convertor comprises several curved sections forming a meander line.
5. The integrated optical circuit as claimed in claim 1, wherein said polarization convertors functions as a lambda over two (λ/2) plate.
6. The integrated optical circuit as claimed in claim 1, wherein said first and second device together form an integrated phased-array multiplexer, said first device comprising a splitter having a plurality of outputs and first waveguides connected to the outputs, said second device comprising a plurality of second waveguides and a combiner having a plurality of inputs, the second waveguides being connected to the inputs, and further wherein said polarization convertor connects one of the first waveguides width one of the second waveguides.
7. The integrated optical circuit as claimed in claim 1, wherein said first and second device together form an integrated Mach-Zehnder interferometer switch, said first device comprising a splitter having a plurality of outputs and first waveguides connected to the outputs, said second device comprising a plurality of second waveguides and a combiner having a plurality of inputs, the second waveguides being connected to the inputs, and further wherein said polarization convertor is combined with an electro-optic switch connecting one of the first waveguides with one of the second waveguides.
Description
BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an integrated optical circuit comprising a first device having an output for emitting radiation with a first state of polarization, a second device having an input for receiving radiation with a second, different state of polarization, and a polarization convertor connected between the output and the input.

The polarization convertor converts radiation guided in a TE mode into radiation guided in a TM mode or vice versa. The TE (transverse electric) mode is a mode in which Ey is the dominant electric component and the TM (transverse magnetic) mode is a mode in which Hy is the dominant magnetic component of the electromagnetic field of the radiation. The y-axis is an axis in the plane of the slab-type substrate of the circuit and perpendicular to the direction of propagation of the radiation. The conversion ratio is the ratio of the amount of radiation in one mode which the convertor converts to radiation in the other mode.

2. Discussion of the Related Art

A circuit as described in the opening paragraph is known from U.S. Pat. No. 5,185,828, disclosing an optical input section for a coherent optical receiver operating on the basis of polarization diversity. The first device is a local radiation source, emitting TE polarized radiation. The polarization convertor changes the state of polarization to 50% TE and 50% TM polarized radiation. The second device comprises a mixer for combining the converted local oscillator radiation with signal radiation, and a detection circuit. The polarization convertor comprises a straight waveguide having a geometric structure consisting of a period sequence of waveguide sections with different widths. Drawbacks of the polarization convertor are that its length is relatively large, of the order of millimeters, and that it has a small bandwidth due to its length.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an integrated optical circuit comprising a relatively small polarization convertor having a large bandwidth.

The object is met by a circuit as described in the opening paragraph, which circuit is characterized according to the invention in that the polarization convertor comprises a curved section of a waveguide. The polarization conversion ratio of the cured section depends on the materials and the geometry of the waveguide. Since a curved waveguide can be made relatively short, the convertor can be made small, resulting in a large bandwidth. The curved section may form an angle of 90 for easy connection to devices of the circuit. The angle may also be smaller or larger than 90.

The conversion ratio increases with decreasing radius of curvature of the curved section. Since a radius of curvature of 100 μm already causes an appreciable conversion ratio, the radius of curvature is preferably smaller than 100 μm.

A strongly curved section of a waveguide may show radiative losses of the guided radiation. The losses may be reduced substantially when according to the invention the curved section of the waveguide has a deep etched waveguide structure.

The conversion originates at locations in the waveguide where the radius of curvature changes. Hence, in cases where the conversion ratio of a single curved section is not sufficient, several curved sections may be concatenated, each transition between sections increasing the conversion ratio of the convertor. The curved sections may be connected by straight waveguide sections. Two curved sections may be connected to form an S shape or a U shape. The sections are preferably arranged to form a meander line, thereby reducing the space in the circuit occupied by the polarization convertor and making it possible to have the input and output of the convertor in line.

In a special embodiment of the circuit according to the invention the polarization convertor functions as a lambda-over-two plate. Such a convertor is especially suitable for use in circuits where the guiding of radiation through a waveguide is polarization dependent. The convertor should preferably be arranged at a symmetric location in the waveguide, such that the polarization dependencies of the waveguide on both sides of the convertor are equal.

A circuit in which the polarization convertor can be used advantageously is a phased-array wavelength multiplexer or demultiplexer. When a polarization convertor functioning as a lambda-over-two plate is arranged in the middle of each waveguide of the phased array, the performance of the phased array will be polarization independent.

It is remarked that a polarization-independent phased-array multiplexer is known from an article by Takahashi et al, published in Optics Letters, 1992, volume 17, no. 7 page 499 to 501. The phased array is made polarization independent by inserting a lambda-over-two quartz plate half-way the waveguides of the array. However, such a multiplexer cannot be integrated on a single substrate. It requires two substrates with the quartz plate in between, thereby generating additional radiation losses in the transitions between the substrates and the quartz plate.

Another advantageous embodiment of the circuit according to the invention is a Mach-Zehnder interferometer switch wherein the switching is realized by electro-optic phase shifters. Since the electro-optic effect is different for TE and TM polarized radiation, the operation of a known interferometer switch will be polarization dependent. According to the invention each of the shifters in the arms of the interferometer comprises a polarization convertor in the form of a curved section of a waveguide. The polarization conversion of the curved waveguide compensates the polarisation dependence of the shifters, making the switch polarization independent.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example and with reference to the accompanying drawings, in which

FIG. 1 shows a phased-array (de)multiplexer,

FIG. 2A shows a cross-section of a waveguide of the phased array of FIG. 1,

FIG. 2B shows a cross-section of a waveguide of the polarization convertor of FIG. 1,

FIG. 3 shows a Mach-Zehnder interferometer switch,

FIG. 4 shows a waveguide having two curved sections, and

FIG. 5 shows the polarization conversion ratio for a single discontinuity of the waveguide of FIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an integrated polarization-insensitive phased-array multiplexer according to the invention. Such a circuit is particularly useful in the field of optical communications to combine optical beams at pump and signal wavelengths in an optical amplifying system. Multiplexers can also be used to increase the transmission capacity of optical fibres by adding closely spaced wavelength bands. Different wavelength bands can be used to provide bidirectional transmission on a single fibre. A demultiplexer can be used to perform the operation opposite to that of a multiplexer, i.e. decomposing an incoming signal into its constituting wavelength bands.

The circuit shown in FIG. 1 comprises two devices 1 and 2 integrated on a single slab-type substrate. The first device 1 has an optical coupler 3, operating as a splitter, for connecting four inputs Ij (j=1, 2, 3, 4) to four outputs Ok (k=1, 2, 3, 4). An input waveguide Uj is connected to each input Ij. Each output Ok is connected to one end of a waveguide Wk. Device 1 has outputs O'k, each of which is connected to the other end of waveguide Wk (k=1, 2, 3, 4). The second device 2 has an optical coupler 4, operating as a combiner, for connecting four inputs I"k (k=1, 2, 3, 4) to four outputs O"m (m=1, 2, 3, 4). Device 2 has four inputs I'k, each of which is connected to one end of a waveguide W'k. The other end of waveguide W'k is connected to the related input I"k. Each output O"m is connected to a waveguide Vm. The couplers may be radiative couplers as known from inter alia European patent application nr. 0 565 308 or multi-mode interference (MMI) couplers as known from international patent application WO 95/22070 (PHN 15 175). The plurality of waveguides Wk and W'k form together the phased-array of the multiplexer. The size of the circuit is of the order of a few square millimeters.

Each output O'k of device 1 is connected to the corresponding input I'k of device 2 by means of a polarization convertor Pk (k=1, 2, 3, 4). Each polarization convertor comprises a waveguide in the form of a meander line with a double U-bend, each bend having a radius of curvature of 50 μm and the convertor having an overall length of about 500 μm. The small size of the convertor can be used advantageously in miniaturizing optical circuits. The size of the polarization convertors is so small, that they can easily be integrated in the phased array. The radius of curvature is chosen such that the polarization convertor operates as a lambda-over-two plate, changing an incoming TE polarization to an outgoing TM polarization and an incoming TM polarization to an outgoing TE polarization. Thus TE polarized radiation at Ok will travel through waveguide Wk of the phased array as TE wave and through waveguide W'k of the phased array as TM wave, whereas TM polarized radiation at Ok will travel through waveguide Wk as TM wave and through waveguide W'k as TE wave. When the waveguides are birefringent, the velocities of propagation of TE and TM polarized radiation will be different. In the phased array according to the invention, incoming radiation always travels half of its path with the velocity of a TE wave and half with the velocity of TM wave. As a result the total travel time of radiation through the two waveguides Wk and W'k is independent of the state of polarization of the radiation at output Ok.

Although FIG. 1 shoes a circuit with four input waveguides Uj and four output waveguides Vm, it is also possible to have a single input waveguide and a plurality of output waveguides, the circuit forming a wavelength demultiplexer. A circuit with a plurality of input waveguides and one output waveguide can be used as a multiplexer, whereas a circuit with one input waveguide and one output waveguide can be used as a comb filter.

FIG. 2A shows a cross-section of a waveguide Wk of the circuit shown in FIG. 1. On a substrate 10 of InP a quaternary InGaAsP guiding layer 11 is grown with a thickness of 0.5 μm outside a rib 12 and a thickness of 0.6 μm inside the rib. A layer 13 of 0.3 μm thick and 1.4 μm wide InP is grown on top of rib 12. FIG. 2B shows a cross-section of a curved section of the waveguide in one of the polarization convertors Pk. The cross-section is similar to the cross-section shown in FIG. 2A, with the difference that a single additional manufacturing step has been applied to the circuit in the form of locally deep etching. As a result, the substrate 10 has the profile of a rib 14 and the quaternary layer 15 has the width or the rib over its entire height. The two waveguides shown in FIGS. 2A and 2B can be easily made on a single substrate, the waveguide of FIG. 2B requiring only a single additional etching step.

FIG. 3 shows a Mach-Zehnder interferometer switch according to the invention. An input waveguide 20 is connected to a first coupler 21, operating as a splitter, having two waveguides 22 and 23 connected to its outputs. The waveguides are connected to a second coupler 24, operating as a combiner, having an output 25 with a waveguide 26 connected to it. Waveguides 22 and 23 form the arms of the interferometer. Part of the waveguides 22 and 23 is overlaid with electrodes 26 and 27 respectively, indicated in the drawing by two thick lines. The electrodes make it possible to change the propagation velocity of the modes in each waveguide electrically. Different electro-optical physical effects can be used to change the refractive indices of the waveguide material, for example carrier injection, carrier depletion or electro-refraction. The electrodes and the part of the waveguides beneath them operate as phase shifters, adapting the phases of the radiation guided in the interferometer arms to the phases required at the entrance of second coupler 24 in order to obtain constructive or destructive interference at output 25.

The sections of waveguides 22 and 23 beneath the electrodes 26 and 27 respectively are curved according to the invention. Since the electro-optical effects are different for TE and TM polarizations, a switch without the curved waveguide sections would be polarization dependent. In the switch according to the invention the curved sections operate as polarization convertors, making the operation of the switch polarization independent.

FIG. 4 shows a curved section of a waveguide for use as a polarization convertor. An InP substrate 30 is provided with a waveguide comprising a quaternary layer 31 having a thickness of 590 nm and a top layer 32 of InP with a thickness of 295 nm; the width of quaternary layer 31 and top layer 32 is 1.4 μm each. The waveguide is designed for radiation with a wavelength of 1.508 μm. The waveguide has in succession a straight section 33, two curved sections 34 and 35, each forming an angle of 90, and another straight section 36. The polarization transformations occur at the transitions 37, 38 and 39 between the sections 33, 34, 35 and 36. The transformation is strongest at transition 38 between the two oppositely curved sections.

FIG. 5 shows the calculated conversion θ at transition 37 as a function of the radius of curvature R of section 34. The conversion is expressed as an angle between the polarization before and after the transition. Hence, a conversion ratio of one, i.e. a complete conversion from TE to TM polarization, corresponds to θ=90. The efficiency of the conversion is apparent from the Figure. A conversion of θ=10 is already achieved in a transition from a straight waveguide section to a section with R=30 μm, and θ=45 for R is about 22 μnm. The beatlength between orthogonal modes of the radiation is of the order of 5 mm for a conversion of θ=45, making the conversion approximately independent of the length of the curved sections. The above calculations have been confirmed by measurements on curved waveguides.

The embodiments of the invention shown in the Figures have curved waveguide sections which are connected to another curved section or to a straight section such that the centreline of the sections is a continuous line. However, the centreline may also show a stepwise, lateral displacement at the transition of two sections in order to match radiation profiles of the sections, thereby reducing radiative losses at the transition.

Patent Citations
Cited PatentFiling datePublication dateApplicantTitle
US4793678 *May 20, 1986Dec 27, 1988Nippon Telegraph And Telephone CorporationFiber optic polarization controller
US5185828 *May 8, 1992Feb 9, 1993Koninklijke Ptt Nederland N.V.Optical waveguide (te,tm) mode converter
US5341444 *Mar 19, 1993Aug 23, 1994At&T Bell LaboratoriesPolarization compensated integrated optical filters and multiplexers
US5475771 *Apr 1, 1994Dec 12, 1995Nec CorporationPolarization splitter haivng an anisotropic optical waveguide
EP0565308A1 *Apr 1, 1993Oct 13, 1993AT&T Corp.Planar lens and low order array multiplexer
WO1995022070A1 *Feb 9, 1995Aug 17, 1995Philips Electronics N.V.Optical device with phased array
Non-Patent Citations
Reference
1"Polarization-Insensitive Arrayed-Waveguide Grating Wavelength Multiplexer On Silicon" H. Takahashi et al, Optics Letters, Apr. 1, 1992, vol. 17, No. 7, pp. 499-501.
2 *Polarization Insensitive Arrayed Waveguide Grating Wavelength Multiplexer On Silicon H. Takahashi et al, Optics Letters, Apr. 1, 1992, vol. 17, No. 7, pp. 499 501.
Referenced by
Citing PatentFiling datePublication dateApplicantTitle
US6275623 *Jul 12, 1999Aug 14, 2001Corning IncorporatedDynamically configurable spectral filter
US6366709 *Dec 22, 1999Apr 2, 2002Chiaro Networks Ltd.Integrated optics beam deflectors and systems
US6542648Jan 24, 2002Apr 1, 2003Chiaro Networks Ltd.Integrated optics beam deflectors and systems
US6556730Jan 24, 2002Apr 29, 2003Chiaro Networks Ltd.Integrated optics beam deflectors and systems
US6690036 *Mar 16, 2001Feb 10, 2004Intel CorporationMethod and apparatus for steering an optical beam in a semiconductor substrate
US6751002May 6, 2002Jun 15, 2004Intel CorporationMethod and apparatus for semiconductor-based integrated polarization modulator/compensator
US6801676Jun 24, 2003Oct 5, 2004Intel CorporationMethod and apparatus for phase shifting an optical beam in an optical device with a buffer plug
US6891653Jun 19, 2002May 10, 2005Intel CorporationMethod and apparatus for steering an optical beam in a semiconductor substrate
US6924849Jun 8, 2001Aug 2, 2005Clarity Visual Systems, Inc.Image projection system with multiple arc lamps and flyseye lens array light homogenizer directing polychromatic light on a liquid crystal display
US6954558Mar 4, 2004Oct 11, 2005Intel CorporationMethod and apparatus for phase shifting an optical beam in an optical device
US7035487Jun 21, 2004Apr 25, 2006Intel CorporationPhase shifting optical device with dopant barrier
US7127129Jun 30, 2005Oct 24, 2006Intel CorporationMethod and apparatus for phase shifting an optical beam in an optical device
US7184613Jan 9, 2006Feb 27, 2007Intel CorporationPhase shifting optical device with dopant barrier
US7242866Aug 29, 2002Jul 10, 2007Intel CorporationMethods and apparatus for switching N optical input signals to M optical outputs
US7280712Aug 4, 2005Oct 9, 2007Intel CorporationMethod and apparatus for phase shifiting an optical beam in an optical device
US7565041 *Oct 26, 2007Jul 21, 2009Infinera CorporationSymmetric optical circuit with integrated polarization rotator
US7792403Sep 8, 2005Sep 7, 2010Infinera CorporationAdiabatic polarization converter
US8213754 *Jun 21, 2007Jul 3, 2012Agency For Science Technology And ResearchOptical splitter, combiner and device
US8450186May 28, 2013Intel CorporationOptical modulator utilizing wafer bonding technology
US20020173058 *Jun 19, 2002Nov 21, 2002Ansheng LiuMethod and apparatus for steering an optical beam in a semiconductor substrate
US20040042792 *Aug 29, 2002Mar 4, 2004Dean Samara-RubioMethods and apparatus for switching N optical input signals to M optical outputs
US20040264828 *Mar 4, 2004Dec 30, 2004Ansheng LiuMethod and apparatus for phase shifting an optical beam in an optical device
US20050244125 *Jun 30, 2005Nov 3, 2005Ansheng LiuMethod and apparatus for phase shifting an optical beam in an optical device
US20050281525 *Jun 21, 2004Dec 22, 2005Samara-Rubio Dean APhase shifting optical device with dopant barrier
US20060120653 *Jan 9, 2006Jun 8, 2006Samara-Rubio Dean APhase shifting optical device with dopant barrier
US20070031080 *Aug 4, 2005Feb 8, 2007Ansheng LiuMethod and apparatus for phase shifting an optical beam in an optical device
US20070280309 *May 23, 2006Dec 6, 2007Ansheng LiuOptical waveguide with single sided coplanar contact optical phase modulator
US20090110344 *Oct 26, 2007Apr 30, 2009Little Brent ESymmetric Optical Circuit with Integrated Polarization Rotator
US20100296775 *Jun 21, 2007Nov 25, 2010Agency For Science, Technology And ReseachOptical splitter, combiner and device
US20110073989 *Mar 31, 2011Haisheng RongOptical modulator utilizing wafer bonding technology
Classifications
U.S. Classification385/11, 385/14
International ClassificationG02B6/34, G02B6/126
Cooperative ClassificationG02B6/12023, G02B6/126, G02B6/12011
European ClassificationG02B6/126, G02B6/12M2P, G02B6/12M2A
Legal Events
DateCodeEventDescription
Feb 20, 1996ASAssignment
Owner name: U.S. PHILIPS CORPORATION, NEW YORK
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNORS:VAN DAM, CORNELIS;HEIDRICH, HELMUT;HAMACHER, MICHAEL;ANDOTHERS;REEL/FRAME:007891/0834;SIGNING DATES FROM 19951120 TO 19951215
Jan 23, 2001FPAYFee payment
Year of fee payment: 4
Mar 16, 2005REMIMaintenance fee reminder mailed
Aug 26, 2005LAPSLapse for failure to pay maintenance fees
Oct 25, 2005FPExpired due to failure to pay maintenance fee
Effective date: 20050826